Computer printers such as ink jet printers normally operate by drawing single sheets of blank media (such as paper or transparent film) from a horizontal stack of sheets. Each sheet is individually drawn or "picked" from the stack, and into the media path of a printer. If no sheets are drawn during an attempted pick, a "no pick" failure has occurred; if two (or more) sheets are picked in an overlapping manner, a "two (or multiple) pick" failure has occurred. In the event of either type of failure, printing may be suspended, media wasted, and a user inconvenienced.
A typical pick mechanism includes a drive or pick roller oriented just above a leading edge of the media stack, for rotation about an axis parallel to the stack edge. The roller has one or more tires spaced along its length. When the leading edge of the stack is lifted, the top sheet contacts the tire surface, and rotation of the roller slides the top sheet off the remaining stack. To help prevent multiple picks, a separator pad opposite one tire rubs on the opposite surface of the picked sheet or sheets. With respect to a media surface, the friction coefficient of the separator is less than that of the pick tire, and greater than that of media, so that a properly picked single sheet proceeds along the media path, while the improper lower sheets of a multiple pick is held by the pad as the upper sheet proceeds alone.
Proper picking action depends largely on the pick force between the upper sheet and the pick tire or tires. If the force is too great, multiple picks are more likely to occur; if the force is too low, "no picks" are more likely. A complicating variable is the changing weight of the media stack as the media tray proceeds from full to empty. Because the force pressing the stack against the pick tire is critical, the mechanism providing this force must provide greater lifting force at the early stages of media depletion than at later stages. At early stages with a full media tray, a smaller displacement is needed to lift the top sheet into contact with the pick tire, as compared at the late stages, when the media tray must be lifted higher, but with less force. This has been addressed in existing printers by the use of conventional springs that provide a linear force or assist proportional to displacement, to neutralize the effects of the media stack weight.
Proper media picking is dependent on many secondary variables, even when the stack weight has been compensated for. As a stack is depleted, the media support plate tilts, and the angle of attack of the top sheet relative to the pick tire changes. Also, as stack height changes, the compressibility of the stack changes, affecting the interaction with the pick tire, and the force required to bend the stack by lifting the leading edge varies in manner believed to be non-linear with respect to stack height. A multitude of other variables affect the optimum pick force (i.e. the compressive force of the top sheet against the pick tire,) but many of these are unknown, and may change widely with different printer designs and configurations in a manner that is difficult to predict. Even when a printer is experimentally characterized by testing different pick forces to determine which force yields the fewest pick failures at each of a selected sample of media fill levels, existing mechanisms lack the controllability or flexibility to provide the desired force as a function of fill level. Such functions may be non linear or otherwise complex.
In addition, the springs used to compensate for media weight are subject to significant manufacturing variations. When a spring is being used throughout a wide range of deflections, particularly at low deflections for a low force, it is vulnerable to variations. For instance, a spring that is deflected by 10% at its lowest used force may provide no force if a dimension or other characteristic is more than minimally outside of tolerances. Springs may be used in a more heavily preloaded condition, but this lacks the capability to compensate proportionally for the weight of a media stack that ranges over a very wide percentage variation. Furthermore, the use of heavy spring tension requires substantial motor force to deflect the springs. This limits the amount of motor torque available for other printer functions. Higher capacity motors may be used, but this increases product cost, size, and weight.
The present invention overcomes the limitations of the prior art by providing a media feed apparatus for a printer, with a motorized media pick mechanism and a media support plate movable toward and away from the pick element, so that a stack of media on the plate may be brought into contact with the pick element. A lifter assembly is continuously movable between a first position and a second position, and has a cam surface supportably contacting the media support plate. A first cam surface portion contacts the support plate when the lifter assembly is in the first position, and a second cam surface portion contacts the support plate when the lifter assembly is in the second position. The support plate may have a protrusion over which the cam surface slides, and the cam may pivoted to provide a selectable lever arm and or contact angle, depending on the point of contact.